Global variables are a great example of bad programming. Global functions too. Generally it is bad to expose the inner workings of your objects. Bad in the full sense of the word.

Ideally, each piece of code should be encapsulated in its own locked box, built of titanium no less than 800 millimetres thick, and buried beneath the ocean floor at its deepest point, at least twelve kilometres from breathable air. It is still possible that subterannean creatures could evolve the ability to withstand the intense presure at such depths, and find ways of penetrating the box and modifying the value of your internal variables. Wiring the titanium box with a nuclear warhead is thus considered best practice.